Shear and Extensional Rheology and Slot-Die Coating Behavior of Polyvinylidene Difluoride Solutions in N -Methyl-2-pyrrolidone

Polyvinylidene difluoride (PVDF) solutions formulated in N -methyl-2-pyrrolidone (NMP) are an important component of lithium-ion battery (LIB) cathode slurry, which is commonly slot-die-coated on the industrial scale. The coating flows involve extensional deformation near stagnation points and within pinching necks. Curved free surfaces, due to the added polymer’s elasticity and extensional rheology, exert a strong influence on coating thickness, stability, and production rates. Despite this well-established application case, there is a lack of careful investigation of the effects of PVDF’s molecular weights and concentrations on its shear and extensional rheology, hindering the prediction of the LIB slurry’s coating stability. To bridge this gap, we formulated solutions of commercial-grade PVDFs with three distinct molecular weights and one bidisperse system. We then measured the steady shear viscosity using torsional rheometry, which revealed shear-thinning response for most of the nondilute solutions. Using specific viscosity as a function of concentration, we identified the overlap concentration for each system and mapped exponents that capture the viscosity increase in dilute and semidilute (unentangled and entangled) solutions. The specific viscosity variation for the three PVDF solutions collapses onto a universal curve when the concentration is scaled to the overlapping concentration. We then characterized the pinching dynamics of PVDF solutions using dripping-onto-substrate (DoS) rheometry, mapped the extensional relaxation time determined from the elastocapillary regime, and extracted the apparent extensional viscosity response. To demonstrate the importance of extensional rheology in slot-die coating, we then identified the coating windows of two concentrated PVDF solutions and showed that for complex fluids like PVDF solutions, including only the shear rheology is inadequate to accurately predict the coating stability. The unique data sets showcase the molecular-weight and concentration-dependent viscoelasticity, strain hardening, and finite extensibility of four commercial-grade PVDF in NMP solutions. We anticipate that the database and protocols will facilitate in selecting better material and processing parameters for LIB slurry coating purposes.